Finagle's Law

Finagle's Law of Dynamic Negatives (also known as Finagle's corollary to Murphy's Law) is usually rendered:

Anything that can go wrong, will—at the worst possible moment

One variant (known as O'Toole's Corollary of Finagle's Law) favored among hackers is a takeoff on the second law of thermodynamics (also known as entropy):

The perversity of the Universe tends towards a maximum.